Wire electrical discharge erosion machines are typically equipped with a device for automatic threading of the wire electrode. An automatic threading device enables complete machining to the carried out in an unsupervised manner. A device of this type is thus of highly substantial significance for wire electrical discharge erosion machines.
The new wire electrode is drawn off from a wire spool and automatically transported through the operating space, and from there subsequently conveyed to the wire disposal. In the operating space, the wire electrode is tensioned between the upper wire-guiding head and the lower wire-guiding head, and is guided in a highly accurate manner. As the wire electrode is introduced the difficulty lies in that in general there must be no threading nozzle or other auxiliary guiding means in the region between the upper and the lower wire-guiding head, because this region has to be free for the relative movements between the workpiece and the electrode. Matters are complicated in that threading in some instances has to be carried out across significant heights of 200 mm and more, that wire electrodes having dissimilar diameter are employed, that the wire electrodes may have dissimilar rigidity, in particular dissimilar wire curvature, or that the wire electrode in the case of a very small diameter displays a thread-like behaviour wherein the wire adheres to guide faces, for example, that dissimilar dielectrics are employed, that at times threading has to be carried out into the existing cutting gap or into a start hole, that the workpiece may have stepped regions, etc.
Wire electrical discharge erosion machines are rarely employed across the entire application spectrum. In any case, the requirements to be met by the wire electrical discharge erosion machine as well as by the automatic threading device are becoming increasingly more demanding in general. In certain application fields there is the requirement for being able to thread into increasingly smaller start holes. While the classic threading jet of water or another fluid that is built up in the wire-guiding head remains the preferred solution in the standard case, the former does meet its limits here. Said threading jet is becoming increasingly unsuitable for the ever smaller start holes, since ever less space remains between the wire and the start hole. A number of solutions have already been proposed in order for this situation to be improved.
In the case of one known threading device according to CH650962, the upper wire-guiding head is guided to the start hole on the workpiece surface, wherein the wire-guiding head in the lower region is provided with a seal such that the wire-guiding head and the workpiece form a pressurized chamber. During the threading procedure the wire will thus be forced to take the path through the start hole, for example. This method is indeed highly effective but may be applied only under ideal conditions. For example, a threading device of this type fails when threading has to be carried out on the periphery of a workpiece or in a stepped region.
The primary tasks of the wire-guiding heads lie in guiding the wire electrode, transmitting the current pulses to the wire, and ensuring purging of the spark gap. The means required therefor should lie as close as possible to the workpiece in order for the best machining results to be able to be achieved. All further tasks, including threading of the wire, are subordinate thereto. To this extent, the improvement potential within the wire-guiding head is limited.
In the case of one further known threading device the wire prior to threading is stretched and severed ahead of the upper wire-guiding head such that the wire is aligned and has a tapered tip. The wire, by means of a closed tubular guide, is guided through the upper wire-guiding head up to the workpiece. The wire thus cannot escape here, and may be reliably threaded. It is disadvantageous in this solution that the wire guide, in order for the tubular guide to be applied, has to be opened or moved, respectively. This requires a special construction of the wire guide, which is expensive and complex since very high requirements are set for the accuracy of the wire guide.
In the case of a solution as is published in EP233297, an external nozzle, composed of two parts, is guided so as to be directly on the upper wire-guiding head, so as to generate a thin threading jet. The external nozzle during machining is in a resting position above the operation region, and is only moved into the threading-operation position while the wire electrode is being set up. However, the fluid jet that may be generated is still relatively thick and moreover rather weak, since there is only little available space for this split threading nozzle.
In the case of a solution according to WO2009157205, an external nozzle that is split in two is likewise guided so as to be directly on the upper wire-guiding head, wherein positioning is performed by means of a pneumatic cylinder, and the two nozzle halves are activated, that is to say opened and closed, by means of a separate drive.
In order for the exact location and position of the nozzle halves relative to one another to be ensured, geometrical features in the form of channels and protrusions are provided on the nozzle halves. In a similar solution according to EP2564967, an external nozzle that is split in two is likewise guided so as to be directly on the upper wire-guiding head, wherein the two halves are held in the open position by means of a spring; the external nozzle is actively closed by moving the external nozzle from the resting position to the operating position and to a detent on the wire-guiding head. In the two last-mentioned cases, the nozzle that is split in two is carried by a solid motion device for moving the jet nozzle between a resting position remote from the wire-guiding head to an operating position contacting the wire-guiding head.
In the case of a solution according to DE3236263A1, a movable guide-tube assembly is guided to the upper wire-guiding head and is aligned with the wire. The wire electrode is pushed into the guide-tube assembly, and the guide-tube assembly per se is guided into the start hole. The wire electrode is pushed further forward and guided into the lower wire-guiding head. Finally, the guide-tube assembly which is provided with a longitudinal slot by way of lateral movement is removed from the guide region.
In the case of a solution according to EP2839917, a movable guide-tube assembly is guided to the upper wire-guiding head. The wire electrode exits the upper wire-guiding head at a slight inclination, and is guided into the guide-tube assembly and into the start hole. The slight inclination guarantees that the wire electrode follows the guide-tube assembly.
With the exception of CH650962, all mentioned solutions are external threading devices. In this context, external means that this device during the normal erosive operation lies outside the wire-guiding head and is moved into the wire-guiding region below the wire-guiding head only during the threading procedure. The reasons for which this threading device is externally disposed include the following: (1) in the case of high requirements, that is to say of small start holes and thin wires, the threading nozzle has to be disposed so as to be very close to the workpiece; however, this space during the erosive operation is also required by the purge nozzle, the wire guide, the power supply, etc, and (2) for “standard” requirements in wire threading, that is to say with start holes >1 mm, a classic threading nozzle integrated in the wire-guiding head suffices; a special threading nozzle for advanced requirements is thus preferably to be seen as an option.
The quality of the threading device may be evaluated in particular by means of the reliability of threading the wire electrode, the duration of the threading procedure, the minimum start hole diameter at which threading may be reliably performed, and the minimum wire diameter that may be reliably threaded.
The installations mentioned above are to some extent very complex and thus expensive. In the case of the minimum start hole diameter and of the minimum wire diameter, the limits are defined by the respective construction.